How to set an onClickListener to a button in kotlin

Question

I am working on a android studio project which is a hybrid of kotlin and java. I am trying to set an onClickListener to a button, however I keep running into compile time errors like the following:

java.lang.RuntimeException: Unable to start activity 
ComponentInfo{com.folioreader.android.sample/com.folioreader.ui.activity.FolioActivity}:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

This is my code:

 private var buttonBookmarks: Button? = null

 //these two are in onCreate
 var buttonBookmarks = findViewById<Button>(R.id.btn_bookmarks)

 buttonBookmarks.setOnClickListener {
        Toast.makeText(this, "Works", LENGTH_LONG).show()
    }

I know that this is an easy question, however, I did not have any experience with Kotlin before starting to work on this so I find it a bit confusing. Some online solutions I have tried did not work so I am asking the question here. What is wrong with my code?

You are missing a `setContentView` – Zun Jun 25 '19 at 08:07 — Zun, Jun 25 '19 at 08:07

Jasurbek · Answer 1 · 2019-06-25T08:14:14.373

4

In Kotlin you do not need findViewById.

btn_bookmarks.setOnClickListener {
        Toast.makeText(this, "Works", LENGTH_LONG).show()
    }

Should work fine by the help of kotlin-synthetic if still error then you should look the ID and make sure they are matched.

Note: To use kotlin-synthetic you should apply plugins like following

apply plugin: 'kotlin-android-extensions'

For more info about extension visit here

edited Jun 25 '19 at 08:14

answered Jun 25 '19 at 08:11

Jasurbek

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That's a good one too, but doesn't really answer the question it's like a "tip", anyways well explained – Skizo-ozᴉʞS ツ Jun 25 '19 at 08:13
This works also, but doesn't really solve the problem. Going to take the tip and remove the findViewByID() method. – Vasko Vasilev Jun 25 '19 at 08:16
Post your activity_main.xml – Jasurbek Jun 25 '19 at 08:33

score 2 · Accepted Answer · edited Jun 07 '21 at 07:02

2

This error

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.folioreader.android.sample/com.folioreader.ui.activity.FolioActivity}:

Says that your activity is not loaded correctly, do you have something like this?

class FolioActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.folio_activity) <-- this one
    }
}

Once you have set the content view you will can use your setOnClickListener as before.

 buttonBookmarks.setOnClickListener {
    Toast.makeText(this, "Works", LENGTH_LONG).show()
 }

edited Jun 07 '21 at 07:02

Ashish

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answered Jun 25 '19 at 08:05

Skizo-ozᴉʞS ツ

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1

If someone is downvoting, please add a reason :) – Skizo-ozᴉʞS ツ Jun 25 '19 at 08:10
1

This worked, this line of code: setContentView(R.layout.folio_activity) was a bit below in the onCreate method. I just placed your code below it and it worked. – Vasko Vasilev Jun 25 '19 at 08:12
@Skizo-ozᴉʞS reason for downvote : context has nothing to with his error. there is problem with `setContentView(R.layout.folio_activity)`and better use `Toast.LENGTH_LONG` without importing another package. – Ashish Jun 25 '19 at 09:45
@Ashish I've edited the question, I removed the context stuff and now is a decent answer, feel free to remove the downvote ;) – Skizo-ozᴉʞS ツ Jun 07 '21 at 07:01

score 0 · Answer 3 · edited Jun 25 '19 at 10:01

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        private var buttonBookmarks: Button? = null

        //these two are in onCreate
        var buttonBookmarks = findViewById<Button>(R.id.btn_bookmarks)

        buttonBookmarks.setOnClickListener {
            Toast.makeText(this, "Works", LENGTH_LONG).show()
        }
    }
}

I just tried your code. It works perfectly. Your syntax seems correct, By seeing your Error

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

only thing is your buttonBookmarks is null reason may be you are either missing the button in your activity's XML file or you are referencing the wrong element.

Comment private var buttonBookmarks: Button? = null too. Hope It helps Thanks

isn't it possible if he removes var from findviewbyid ? and just provide ContentView layout name to his activity ? — Ashish, Jun 25 '19 at 09:43
yes you can remove var and directly call setOnClickListener on findViewById — CodeWithVikas, Jun 25 '19 at 12:57

How to set an onClickListener to a button in kotlin

3 Answers3